The complement system is a complex enzyme cascade made up of a series of serum glycoproteins, that normally exist in inactive, pro-enzyme form. Two main pathways, the classical and the alternative pathway, can activate complement, which merge at the level of C3, where two similar C3 convertases cleave C3 into C3a and C3b.
Macrophages are specialist cells that have developed an innate capacity to recognize subtle differences in the structure of cell-surface expressed identification tags, so called molecular patterns (Taylor, et al., Eur J Immunol 33, 2090-2097 (2003); Taylor, et al., Annu Rev Immunol 23, 901-944 (2005)). While the direct recognition of these surface structures is a fundamental aspect of innate immunity, opsonization allows generic macrophage receptors to mediate engulfment, increasing the efficiency and diversifying recognition repertoire of the phagocyte (Stuart and Ezekowitz, Immunity 22, 539-550 (2005)). The process of phagocytosis involves multiple ligand-receptor interactions, and it is now clear that various opsonins, including immunoglobulins, collectins, and complement components, guide the cellular activities required for pathogen internalization through interaction with macrophage cell surface receptors (reviewed by Aderem and Underhill, Annu Rev Immunol 17, 593-623 (1999); Underhill and Ozinsky, Annu Rev Immunol 20, 825-852 (2002)). While natural immunoglobulins encoded by germline genes can recognize a wide variety of pathogens, the majority of opsonizing IgG is generated through adaptive immunity, and therefore efficient clearance through Fc receptors is not immediate (Carroll, Nat Immunol 5, 981-986 (2004)). Complement, on the other hand, rapidly recognizes pathogen surface molecules and primes the particle for uptake by complement receptors (Brown, Infect Agents Dis 1, 63-70 (1991)).
Complement consists of over 30 serum proteins that opsonize a wide variety of pathogens for recognition by complement receptors. Depending on the initial trigger of the cascade, three pathways can be distinguished (reviewed by (Walport, N Engl J Med 344, 1058-1066 (2001)). All three share the common step of activating the central component C3, but they differ according to the nature of recognition and the initial biochemical steps leading to C3 activation. The classical pathway is activated by antibodies bound to the pathogen surface, which in turn bind the C1q complement component, setting off a serine protease cascade that ultimately cleaves C3 to its active form, C3b. The lectin pathway is activated after recognition of carbohydrate motifs by lectin proteins. To date, three members of this pathway have been identified: the mannose-binding lectins (MBL), the SIGN-R1 family of lectins and the ficolins (Pyz et al., Ann Med 38, 242-251 (2006)) Both MBL and ficolins are associated with serine proteases, which act like C1 in the classical pathway, activating components C2 and C4 leading to the central C3 step. The alternative pathway contrasts with both the classical and lectin pathways in that it is activated due to direct reaction of the internal C3 ester with recognition motifs on the pathogen surface. Initial C3 binding to an activating surface leads to rapid amplification of C3b deposition through the action of the alternative pathway proteases factor B and factor B. Importantly, C3b deposited by either the classical or the lectin pathway also can lead to amplification of C3b deposition through the actions of Factors B and D. In all three pathways of complement activation, the pivotal step in opsonization is conversion of the component C3 to C3b. Cleavage of C3 by enzymes of the complement cascades exposes the thioester to nucleophilic attack, allowing covalent attachment of C3b onto antigen surfaces via the thioester domain. This is the initial step in complement opsonization. Subsequent proteolysis of the bound C3b produces iC3b, C3c and C3dg, fragments that are recognized by different receptors (Ross and Medof, Adv Immunol 37, 217-267 (1985)). This cleavage abolishes the ability of C3b to further amplify C3b deposition and activate the late components of the complement cascade, including the membrane attack complex, capable of direct membrane damage. However, macrophage phagocytic receptors recognize C3b and its fragments preferentially; due to the versatility of the ester-bond formation, C3-mediated opsonization is central to pathogen recognition (Holers et al., Immunol Today 13, 231-236 (1992)), and receptors for the various C3 degradation products therefore play an important role in the host immune response.
C3 itself is a complex and flexible protein consisting of 13 distinct domains. The core of the molecule is made up of 8 so-called macroglobulin (MG) domains, which constitute the tightly packed α and β chains of C3. Inserted into this structure are CUB (C1r/C1s, Uegf and Bone mophogenetic protein-1) and TED domains, the latter containing the thioester bond that allows covalent association of C3b with pathogen surfaces. The remaining domains contain C3a or act as linkers and spacers of the core domains. Comparison of C3b and C3c structures to C3 demonstrate that the molecule undergoes major conformational rearrangements with each proteolysis, which exposes not only the TED, but additional new surfaces of the molecule that can interact with cellular receptors (Janssen and Gros, Mol Immunol 44, 3-10 (2007)).
Age-related Macular Degeneration (AMD) is the leading cause of blindness in the elderly worldwide. AMD is characterized by a progressive loss of central vision attributable to degenerative and neovascular changes in the macula, a highly specialized region of the ocular retina responsible for fine visual acuity. Recent estimates indicate that 14 million persons are blind or severely visually impaired because of AMD. The disease has a tremendous impact on the physical and mental health of the geriatric population and their families and is becoming a major public health burden.
New discoveries, however, are beginning to provide a clearer picture of the relevant cellular events, genetic factors, and biochemical processes associated with early AMD.
Factor B is a tightly regulated, highly specific serine protease. In its activated form, it catalyzes the central amplification step of complement activation to initiate inflammatory responses, cell lysis, phagocytosis and B-cell stimulation (Carroll et al., Nat. Immunol. 5:981-986 (2004)). factor B is activated through an assembly process: it binds surface-bound C3b, or its fluid-phase counterpart C3(H2O), after which it is cleaved by factor B into fragments Ba (residues 1-234) and Bb (residues 235-739) (Fishelson et al., J. Immunol. 132:1430-1434 (1984)). Fragment Ba dissociates from the complex, leaving behind the alternative pathway C3 convertase complex C3b-Bb, which cleaves C3 into C3a and C3b. This protease complex, is intrinsically instable. Once dissociated from the complex, Bb cannot reassociate with C3b (Pangburn et al, Bochem. J. 235:723-730 (1986)).
The proenzyme factor B consists of three N-terminal complement control protein (CCP) domains, connected by a 45-residue linker to a VWA domain and a C-terminal serine protease (SP) domain, which carries the catalytic center. Striking differences from other serine proteases are observed in the active center of factor B (Xu et al., Immunol. Rev. 180: 123-135 (2001)). The catalytic triad residues Asp102, His57 and Ser195 (chymotrypsinogen numbering has been used for the serine protease domain), and the three residues forming the non-specific substrate binding site, Ser-Tip-Gly214-216, have typical conformations. However, the oxyanion hole displays an inactive conformation similar to that observed in certain zymogens. This is primarily due to an inward orientation of the carbonyl oxygen of Arg192, the backbone of which, along with those of Cys191, Gly193 and Asp194, adapts a single turn 310-helix conformation. The carbonyl oxygen of Arg192 forms an H bond with the amide group of Ser195, thus reducing the positive charge that characterizes active oxyanion holes. This inactive configuration of the oxyanion hole is not compatible with the high catalytic activity expressed by the C3 convertase. Accordingly, a conformational change leading to realignment of the structural elements of the oxyanion hole must be induced by the co-factor, the substrate, or both.
The level of factor B is relatively high in the plasma is ˜1665 nM, but low in the eye ˜29 nM). Therefore, it forms an attractive target for antibody therapy. The present invention provides anti-factor B antibodies for the prevention and treatment of AMD and other complement-associated eye conditions.